474 EDWARD STEIDTMANN 



While the evidence for the solubility of sihca under weathering may 

 be satisfactory, there is very httle to show that weathering may 

 yield siliceous deposits of anything like the thickness, high sihca 

 content, and freedom from detrital sihca and clay as the Lake 

 Superior sedimentary iron formations. 



Moore beHeves that the Keewatin was a period of inclosed basins 

 and imperfect drainage, conditions especially favorable to the solu- 

 tion, transportation, and deposition of iron. The long period of 

 freedom from intense vulcanism, which he beheves is imphed by the 

 thickness of the iron formation, removes the most apparent diffi- 

 culty in assuming the development of a luxuriant flora, a condition 

 conducive to the solution of iron and sihca, and a selective retention 

 of the complementary elastics on the land areas. Unless it ( an be 

 shown that the elastics associated with the iron formation are pyro- 

 clastic in origin, it is obvious that a great deal of rock was dis- 

 integrated by weathering during the Keewatin. With due regard 

 for the facts which may be interpreted as evidence for weathering 

 and magma tic contribution, Moore concludes that the materials of 

 the iron formation were supphed to inclosed basins by weathering 

 action under the influence of plant life, and by heated igneous rocks 

 coming in contact with the waters. 



Mclnnes' reports that the pre-Cambrian rocks of the region 

 between N. lat. 50° 10' and N. lat. 55° 10', and between W. long. 

 86° and W. long. 90°, include Keewatin diorites, diabases, and 

 chloritic and hornblende schists, and coarse conglomerates, the 

 latter probably Huronian, and Laurentian biotite granite gneisses. 



Parsons^ writes detailed descriptions of the gold prospects and 

 mines of the Lake of the Woods, Manitou, and Dryden areas of 

 northwestern Ontario. 



Seelye^ states that the Helen limonite ore body at Michipicoten 

 is bounded by cherty iron carbonate, chert, and a diabase dike. 



' W. Mclnnes, "Report on a Part of the Northwestern Territories Drained by the 

 Umisk and Ottawapiskat Rivers," Canada Geol. Survey, 1910, 54 pp., S pis., i map. 



2 A. L. Parsons, "Gold Fields of Lake of the Woods, Manitou and Dryden," Ont. 

 Bur. Mines, (1911) pp. 158-98, 26 figs., 4 pis. 



3 R. W. Seelye, "The Helen Mine, Michipicoten, Ontario," Jour. Can. Min. Inst., 

 XIII (1910), 121-34. 



